1. Field of the Invention
The present invention relates to an optical amplifier for amplifying a plurality of channels of signal light having respective wavelengths different from each other in an optical waveguide supplied with pumping light, and an optical communication system including the same.
2. Related Background Art
Optical communication systems transmit a large volume of information by propagating multiplexed signal light through an optical fiber transmission line. As a signal wavelength band, such optical communication systems have already employed C band (1530 nm to 1565 nm), whereas the use of L band (1565 nm to 1625 nm) has been under study. For achieving a greater volume, the use of S band (1460 nm to 1530 nm) as a signal wavelength band has also been under consideration.
The optical communication systems employ optical amplifiers for amplifying the signal light. Employed as an optical amplifier capable of amplifying the signal light in C or L band is EDFA (Erbium Doped Fiber Amplifier) in which an optical amplification fiber having a light-guiding region doped with Er (erbium) element (EDF: Erbium Doped Fiber) is used as an optical amplification medium. By supplying pumping light (in the wavelength band of 0.98 μm or 1.48 μm) to the EDF, the EDFA can amplify the C or L band of signal light propagating through the EDF.
On the other hand, as an optical amplifier capable of amplifying the S band of signal light, TDFA (Thulium Doped Fiber Amplifier), in which an optical amplification fiber having a light-guiding region doped with Tm (thulium) element (TDF: Thulium Doped Fiber) is used as an optical amplification medium, has been under study. By supplying pumping light (in the wavelength band of 0.8 μm, 1.05 μm, 1.2 μm, 1.4 μm, or 1.55 to 1.65 μm) to the TDF, the TDFA can amplify the S band of signal light propagating through the TDF.
While the EDFA utilizes the three-level type transition (at ground level, 4I13/2 level, and 4I15/2 level) of Er ion, the TDFA utilizes the four-level type transition (at ground level, 3H4 level, 3F4 level, and 3H6 level) of Tm ion. As a consequence, the TDFA can modify the magnitude of gain without deteriorating the gain flatness even when the length of TDF is held constant (see, for example, literature 1, i.e., T. Sakamoto, et al., “Gain-equalized thulium-doped fiber amplifiers for 1460 nm-based WDM signals”, OAA'99, Technical Digest, WD2 (1999)).